1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing glass substrate suitable for storage medium such as magnetic disk, storage medium and glass substrate therefor manufactured using the method or apparatus.
2. Description of the Related Art
A glass substrate for a magnetic disk has hitherto been manufactured with a lapping/polishing method where a substrate of a predetermined size is lapped and polished so as to obtain a smooth surface. However, in recent years, it has been desired to increase the smoothness of the substrate. Thus, disadvantageously, the lapping/polishing method requires a large number of steps and increases the price of a glass substrate product.
On the other hand, a molding method, where glass material is heated, molded, and cooled to precisely transfer die molding surfaces to the material, does not require any post-working and thus does not require high costs and is highly productive. Therefore, in the field of manufacture of optical elements, many efforts have already been made to improve molding methods and to put them to practical use.
A conventional pressing process used to manufacture optical elements is generally executed as follows. A glass material is set between an upper and lower dies. In order to prevent the dies from being oxidized, the atmosphere of a molding chamber is replaced with an inert gas such as a nitrogen gas, with the molding chamber accommodating the die and the glass material. Subsequently, a heater (an induction heater, a lamp heater, a cartridge heater, or the like) is used to heat the dies and the glass material. Once the temperature reaches a predetermined value, the upper and lower dies are used to press the glass material. Finally, the dies are cooled, and a product is then removed.
For such pressing of an optical element, to prevent defects or oxidation of the dies caused by collected air, a method has been proposed which presses a material in a reduced pressure atmosphere and which blows an inert gas against the dies (refer to, for example, Japanese Patent Application Laying-open No. 10-330121 (1998)).
However, the manufacture of glass substrate for magnetic disks using a molding method involves problems different from those of the pressing of an optical element.
Specifically, while a substrate for a hard disk medium have an area larger than those of optical elements, the waviness of the surface of the substrate has to be minimized (the surface be as flat as possible) notably in a circumferential direction, and the dimensional accuracy of the inner and outer diameters of the substrate be increased. For example, 2.5-inch disks may need to have a flatness of 4 μm or less. Such a requirement is based on the need of improving the flying capability of a magnetic head particularly during a high speed rotation.
With the conventional method of blowing a gas against dies for cooling, it is difficult to cool the dies at a uniform temperature which, in turn, may lead to deformation of the substrate, making it difficult to provide a suitably flat substrate.
On the other hand, a method has been proposed where glass substrates for magnetic disks are manufactured with heating, pressing, and cooling steps performed in chambers separate from one another (refer to, for example, Japanese Patent Application Laying-open No. 2000-351635).
However, when a substrate is conveyed between the number of chambers during a single molding process, a chuck mark may be left on the substrate, and also, the substrate may be distorted.
In view of the above problems of the prior arts, it is an object of the present invention to provide means for suitably producing a high-quality substrate for a storage medium with a molding method.